Information can be found here:
http://office.microsoft.com/en-gb/outlook-help/i-get-warnings-about-a-program-accessing-e-mail-address-information-or-sending-e-mail-on-my-behalf-HA001229943.aspx

Make sure to read the link provided before making the following changes.

The following instructions are for Outlook 2013.

1. Go to 'File' and click 'Options'.
file.png

2. Click 'Trust Center' on the left and then click 'Trust Center Settings' on the right.
options.png

3. Click 'Programmatic Access' on the left and then make the appropriate changes.
trustcenter.png